The present invention relates to an optical scanning apparatus used in an image forming apparatus such as a copying machine, facsimile, printer, and the like, and also to an image forming apparatus having such an optical scanning apparatus. Particularly, the present invention relates to an optical scanning apparatus and an image forming apparatus used for forming a color image by superimposing toner images of plural colors.
In an image forming apparatus that uses a Carlson process, electrostatic latent images are formed on a rotating photosensitive drum used as an image carrier, and development and transfer of the electrostatic latent images are carried out subsequently. In order to carry out such formation of the electrostatic latent images, the image forming apparatus is equipped with an optical scanning apparatus that irradiates an optical beam on the surface of the photosensitive drum in the form of a laser beam according to the image information to be recorded.
In order to obtain high-quality images in such an image forming apparatus, it is necessary to scan the photosensitive drum by the optical beam, which is produced by an optical scanning apparatus, with high precision.
Particularly, in the case the image forming apparatus is a multicolor image forming apparatus that forms color images by arranging plural photosensitive drums in a feeding direction of the image transfer body in the form of plural image-forming stations of respective colors, such that toner images of the respective colors formed in such image-forming stations are superimposed on the image transfer body, there easily occurs degradation of image quality such as color misalignment or incorrect color, unless the irradiation position, and hence the scanning position, of the optical beam is aligned exactly in each of the plural photosensitive drums. Thus, an optical scanning apparatus is required to scan the photosensitive drum with the optical beam exactly.
Further, in the case there exists a variation in the duration between the formation of the electrostatic latent images and the transferring of the toner images to the transfer body or there exists a variation in the spacing between the photosensitive drums of different colors as a result of decentering of the photosensitive drums or as a result of the diametric variation of the photosensitive drums, or in the case there exists a variation in the moving speed or meandering in any of the toner image transfer body, which may be a toner image transfer belt, or a transportation belt that transports the recording sheet, there arises the problem of degradation of the image quality such as color misalignment or wrong color caused by the register error of the toner images of the respective colors.
Conventionally, such register error has been compensated for, irrespective of whether it is caused by the optical scanning apparatus or by other reasons, by detecting the sub-scanning position periodically between the image formation jobs by using a register error detection pattern recorded on the toner image transfer body, and by aligning the timing of the start of writing, such that the start line position of recording is aligned on the toner image transfer body (Patent Reference 1 or Patent Reference 2).
Further, there is proposed a process of compensating for a skew of the scanning line on a photosensitive drum in the Patent Reference 3 by tilting a reflection mirror extending in the scanning direction of the optical beam and used for reflecting the scanning optical beam toward the photosensitive drum, about an end point thereof. Further, there is proposed a process of compensating for a skew of the scanning line in the Patent Reference 4 by tilting a scanning lens.
With regard to the problem of curve of the scanning line caused by the error at the time of manufacturing of the scanning lens or misalignment of the scanning lens, the Patent Reference 5 teaches the use of rectification of the scanning lens, which has the lens power in the sub-scanning direction, at the time of manufacturing of the image forming apparatus.
Particularly, it is very important for the recording of high-quality color images in the optical scanning apparatus that includes plural optical elements such as lenses and mirrors in addition to the optical source such that the optical beam produced by the optical source scans the photosensitive drum after being processed by such plural optical elements, in that such plural optical elements are aligned exactly and exact scanning of the optical beam is made over the photosensitive drum.
Thus, it has been practiced conventionally in the multicolor image forming apparatus according to the Patent Reference 7 or Patent Reference 8 to scan the optical beams from the optical sources of the respective colors simultaneously by using a single polygonal mirror and provide cooperating plural deflection mirrors such that the optical beams of the respective colors are directed to the corresponding photosensitive drums. Thereby, in order to hold the foregoing plural deflection mirrors with high precision such that exact scanning is made in each of the photosensitive drums of the respective colors, there is proposed a construction that holds these deflection mirrors on a common housing of the optical scanning apparatus.
Further, with regard to the multicolor image forming apparatus, the Patent Reference 9 or Patent Reference 10 discloses the construction in which an optical unit is provided for each of the multiple colors and hold the optical units on a common side plate frame with positioning for maintaining the mutual positional relationship between the scanning lines of the respective colors.
In the multicolor image forming apparatuses of the type in which plural image formation stations of respective colors are disposed along the transportation direction of the toner image transfer body for superimposing color toner images thereon, it is thus necessary to align the positions of the color toner images transferred from the respective photosensitive drums to the toner image transfer body exactly for avoiding color misalignment or recording of wrong color images. Further, deviation of the scanning position causes also the degradation of images even in the case of carrying out formation of monochromatic images.
Thus, in the conventional optical scanning apparatuses, it has been practiced to increase the rigidity of the housing of the optical scanner by using a metal plate for this purpose or by forming the optical scanner housing by using die-cast aluminum, as disclosed in the Patent Reference 6, so as to avoid deviation of the scanning position caused by mechanical vibration or for to enable high-precision positioning.